During the summer, Albus Dumbledore enlists Harry’s help to persuade retired professor Horace to return to Hogwarts to teach. Harry is also told that he has inherited all of Siriuss possessions. Harry then spends the remaining holiday at with the Weasleys and Hermonie. Harry, Ron, and Hermione receive their O.W.L. results. Hermione receives top "Outstanding" marks in all her subjects with the exception of Defence Against the Dark Arts. Harry is very pleased to discover that he was the only one out of the three of them to get an "Outstanding" in Defence Against the Dark Arts.
As school begins, Snape is announced as the new Defence Against the Dark Arts while Slughorn replaces him as the Potions teacher. Slughorn lends them old Potions textbooks until they buy their own. On the first Hogsmeade visit, Katie Bell is seriously injured while carrying a cursed necklace through Hogsmeade under the Imperius Curse. In another incident, Ron accidentally drinks poisoned mead intended for Dumbledore. Harry reacts by administering the Bezoar he had submitted previously as an assignment to Slughorn. When Harry finds Malfoy sobbing in a boys bathroom on the sixth floor, they hurl curses at each other. Snape arrives and saves Malfoy. He attempts to re-possess the Half-Blood Prince's Potions book, but Harry hands him "Roonil Wazlib's" copy. Harry receives detention, causing him to miss the Quidditch finals. Nonetheless, Gryffindor wins the Cup, and during the victory celebration, Harry's suppressed feelings for Ginny are revealed when he spontaneously kisses her, and she and Harry begin dating.
Dumbledore, meanwhile, has located another Horcrux and asks Harry to accompany him in retrieving it. Harry and Dumbledore apparate to a secret cave. Dumbledore was greatly weakened due to the mysterious liquid he had to drink in order to acquire the locket. Returning to Hogsmeade, Harry and Dumbledore see Lord Voldemort's Dark Mark. They fly to the
As members of the Order and the few from Dumbledore's Army battle Voldemort's followers in the castle below, Death Eaters appear in the tower and urge Draco to fulfil his mission, killing Dumbledore, but Draco hesitates. Snape arrives and killed Dumbledore. Malfoy escapes as Snape duels Harry. During the duel, Snape reveals that he is the Half-Blood Prince.
The school year ends abruptly with Dumbledore's funeral. Professor Slughorn replaces Snape as the head of Slytherin House. Hogwarts is rumoured to close down due to the murder of Dumbledore. Harry decides to leave school in search for the remaining Horcruxes. Ron and Hermione vow to accompany him, while Harry ends his relationship with Ginny to protect her from Voldemort.
"You dare use my own spells against me, Potter? It was I who invented them - I, the Half Blood Prince!"
Taken from Harry Potter and the Half-Blood Prince by J.K. Rowling